The present invention relates to semiconductor structures.
In a typical manufacturing process, semiconductor dies are encapsulated into a protective package (e.g., a ceramic or plastic package) and attached to a printed circuit board (PCB).
To reduce system size and signal delays, dies can be bonded directly to a PCB before encapsulation (so-called flip-chip packaging). Dies can also be stacked on top of each other before encapsulation (this is called xe2x80x9cvertical integrationxe2x80x9d).
Another technique is described in an article by H. Kanbach et al., xe2x80x9c3D Si-on-Si Stack Packagexe2x80x9d, 1999 International Conference on High Density Packaging and MCMs, pages 248-253. Individual dies are attached to a silicon substrate (a xe2x80x9cfloorxe2x80x9d) in a flip-chip manner. The floors, with dies attached, are stacked on top of each other, and the whole stack is mounted on a PCB. The stack is mounted so that the dies of each floor are located under the floor. The dies are bonded to contact pads formed on the flat bottom surface of the floor. The floor has a cavity in its upper surface to accommodate the dies bonded to the next floor above. The dies attached to different floors are interconnected by means of metallized vias etched through the floors at locations away from the cavities. According to the article, a large number of interconnections can be provided between the dies attached to different floors.
The present invention provides structures and fabrication techniques related to semiconductor dies at least partially located in a cavity formed in a substrate. In some embodiments, a metallized via passes through the substrate and opens into the cavity. A die is bonded to a contact pad formed at the end of the via in the cavity. Dies can be bonded to the substrate on both sides, i.e., some dies can be bonded in the cavity, and other dies can be bonded to the substrate""s surface opposite to the cavity. The substrate can be mounted on a PCB so that the substrate and the PCB enclose the cavity. The metallized vias do not pass through the thickest portion of the substrate because they terminate at the cavity. Hence, the metallized vias are easier to fabricate. Also, the signal path between the opposite sides of the substrate is shorter. The cavity sidewalls, which laterally surround the cavity, can be made wide to make the structure rigid. This is especially beneficial if the PCB is flexible. The wide sidewalls can also provide support for wire bonding performed to attach bond wires to the substrate""s surface opposite to the cavity.
The substrate may have a pressure relief passage connecting the cavity with the substrate""s surface other than the surface in which the cavity is formed. The passage can be a through hole, or it can be a groove etched in the substrate""s surface in which the cavity is formed. The passage provides an escape or pressure relief path for material filling the cavity. The material can be gaseous (e.g. air), solid, semi-solid (soft), or in any phase. The passage can also be used to introduce the material into the cavity. The cavity can thus be easily filled with the material.
The invention is not limited to the embodiments described above. For example, the metal in the vias can be replaced with doped polysilicon or some other conductive material. The pressure relief passages may be non-horizontal passages, and may be incorporated into prior art structures such as described in the aforementioned article by Kanbach et al. Other features of the invention are described below. The invention is defined by the appended claims.